1. Field of the Invention
The present invention relates to dishwashing equipment and, more particularly, to an improved dishwashing machine in which excess heated washing solution is used to preheat the incoming rinsing water as the excess solution is routed to the drain.
2. Prior Art
In a typical commercial dishwasher, fresh water is first heated and then circulated through the rinsing system to rinse the foodware items. In order to conserve water and heat in such dishwashers, this heated rinsing water is then collected in a sump, supplementing the water already therein which along with detergent forms the washing solution. In this fashion, the solution in the sump need only be heated a small amount in order to reach the requisite sanitizing temperature to replace the heat lost to the foodware items. Excess washing solution now in the sump overflows from the sump into an overflow standpipe through which it is drained from the dishwasher as waste water.
A large amount of the energy used to operate commercial dishwashers of this type is directed to heating the fresh water flowing into the rinsing system and maintaining the temperature of the washing solution as it is held in the sump. A substantial amount of heat energy is lost in the heated solution which overflows from the sump to the drain and out of the dishwasher. Some devices have been proposed in the prior art to recapture the heat energy stored in the heated water within the sump of a commercial dishwasher and thereby reduce the amount of energy required to operate the dishwasher. One method has been to use the heated water from the sump to preheat the fresh water used for rinsing prior to its entering a water heater from which heated rinsing water is supplied to the dishwasher so that the water heater does not have to use a large amount of energy to heat the rinsing water to the required temperature.
For example, in U.S. Pat. No. 1,332,712, the line which supplies fresh water for rinsing to the water heater passes through the sump which contains heated water used for washing. The section of line passing through the sump is coiled to increase the surface area of the line in contact with the solution.
The primary disadvantage is that the heat energy being removed from the washing solution to preheat the rinsing water is being removed from washing solution contained within the sump. Since the washing solution in the sump must be maintained at a temperature sufficient to sanitize the foodware items, heat energy must be added to the water in the sump from an external source to compensate for the heat removed by the fresh water passing through. Thus, there is a trade-off which might not result in any actual savings in energy for this type of device.
A secondary disadvantage of such heat recovery systems is that the washing solution typically contains food particles of varying sizes which may deposit on the coils of the rinse line and thereby impair the heat transfer capability of the coil. This is especially true if a heat exchanger having a multiplicity of finely spaced fins is used instead of a coil.
A second example is found in U.S. Pat. No. 4,156,621. Used heated rinsing solution from the sump of a commercial dishwasher is first pumped into holding tanks. By use of auxiliary pumps the solution is then pumped through reverse osmosis and softening units for regeneration back to reusable quality and therefrom pumped back to the holding tanks. At the holding tanks, the regenerated water passes through immersed heat exchanger coils through which the cooler regenerated water absorbs heat energy from used heated rinsing solution stored in the tanks. The regenerated and now preheated rinsing water then flows to a booster heater from which it is pumped for use in rinsing the foodware items in the dishwasher.
One drawback to the use of this system solely for heat energy recovery from used heated rinsing water is in its implication that temporary holding tanks be provided for receiving the used heated rinsing water. The additional investment in equipment and space for the equipment alone would greatly diminish the viability of this system as a feasible approach to energy recovery. Another disadvantage is the low heat transfer efficiency of immersed coil type heat exchangers.
These two examples of prior art attempts at heat energy recovery from used heated dishwasher water are not seen as satisfactory approaches to the goal of energy convervation. Accordingly, the need still exists for a viable heat recovery system for a dishwasher in which heat energy contained in the used water or washing solution being drained from the dishwasher is transferred at high efficiency rates to the cooler fresh water to be used for rinsing in the dishwasher so that minimal supplemental energy is required to be added to the rinsing water to heat it to the requisite temperature. Such a system should not remove heat from the washing solution held within the sump in order to heat the rinsing water.